Method of seismic surveying



Dec. 30, 1941. c. H. DIX

METHOD OF SEISMIC SURVEYING 2 Sheets-Sheet 1 Filed Sept. 8, 1941 /5 A JR FIG. 2

DISTANCE-*- MSIR CHARLES HEWITT DIX p INVENTOR @TTORNEY Dec. 30, W41. c. H. DIX

METHOD OF SEISMIC SURVEYING Filed Sept. 8, 1941 2 Sheets-Sheet 2 CHARLES HEWITT DIX vii INVENTOR W ATTORNEY Patented Dec. 30, 1941 2,267,858 METHOD OF SEISMIC SURVEYING Charles HewittDix, Pasadena, Calif., assignor to Socony-Vacuum Oil Company, Incorporated, New York, N. Y., a corporation of New York Application September 8, 1941, Serial No. 410,113

2 Claims.

This invention relates to geophysical prospecting and more particularly to a refraction profile method of seismic surveying.

In recent years the refraction method of seismic geophysical investigation has fallen into disfavor because of the high cost of practicing it. But the reflection methods which have largely replaced it are not wholly satisfactory; it is often difficult or impossible to select the significant fiection from among the many disturbances which appear on the seismograph records, and, when the geological interface to be investigated is not perfectly sharp and definite, useful refiections are frequently unobtainable.

It is, therefore, an object of my invention to reduce the cost of refraction profile methods of seismic surveying.

More specifically, it is an object of my invention to eliminate the necessity of running tests over the same ground in both directions in order to obtain reversed profile data.

A further object of my invention is to increase the completeness and continuity of refraction profile data obtainable at reasonable cost.

Other objects and advantages of my invention will become apparent from the following detailed description, when considered with the accompanying drawings in which:

Fig. 1 illustrates a fragmentary section of the earths surface showing a plurality of substrata and the location of a shot point as well as the detecting stations along the surface;

Fig. 2 is a chart plotted from data such as might be obtained at the detecting stations shown in Fig. 1;

Fig. 3 is a diagram illustrating the practice of the present invention, in which diagram a sectional representation of the earths surface and a chart of plotted data are combined; and

Fig. 4 is a chart illustrating an alternative method of plotting the data obtainable by my method. v I

Referring to Figs. 1 and 2, I will first briefly review the refraction profile method as generally practiced heretofore, in order that my invention may be more'readily understood. Fig. 1 illustrates in section the surface of the earth in and a series of layers ll, l2, and I3, each layer having an average seismic wave velocity distinctly greater than that of the layer next above. The shot point is shown at I 4, and a series of recording seismometers, all lying on a substantially straight line on the surface, is indicated at l5l5. The seismometers are customarily spaced at equal intervals, for example 250 meters apart. A heavy charge of dynamite or the like is buried at M and exploded by an electric cap, and it is arranged that the fusing of the resistance wire in the cap actuates an electrical impulse which is transmitted, by either radio or wire, to the various seismometer stations. Thus a record of the instant of the explosion is obtained at each station.

The seismic pulse generated by the explosion may be regarded as travelling along various ray paths, a few of which are indicated in Fig. 1. It will be noted that the rays are shown as slightly curved; this is because of the usual tendency of seismic velocity to increase continuously with increasing depth within each layer, which produces the effect of curved paths by refraction. Ray I6 is an example of a ray of the so-called direct pulse. i. e., of the pulse which travels entirely in the first layer, ll. Another ray path is illustrated at l1--l8l9; in this path, partial ray I1 meets stratum I2 at such an angle that it is refracted along path l8, which passes through stratum I2 substantially parallel to its upper boundary. Upon emerging into upper layer H, the ray is again refracted and passes along l9 until it reachesthe surface. Still another type of ray path is represented at 20 2 l22-2324. In this path, the ray penetrates both layers H and I2 and travels just below the upper boundary of layer l3, as shown by partial ray 22. The ray paths which lie in part in the deeper layers represent the paths of the re-' fracted pulses" which are the particular objects of study in the art of refraction seismometry.

The nature of the data obtained by recording seismometers l5l5 is illustrated in Fig. 2. This figure is a chart in ordinary Cartesian coordinates, in which'the abscissae representdistances from the shot point [4 along the line of survey and the ordinates represent time elapsed after the instant of the explosion. The locations of the seismometer stations are indicated by the small triangles [5a along the X axis, and the tween layers II and I2. Points 32 and 33-33 correspond to the arrivals of the pulse which penetrates into the third layer, I3, and they lie upon still another line, 34.

Points 25-25, 29-43, and 33-33, which are indicated by small circles, are referred to as primaries, since these represent the first vibrations recorded by the seismometers. The other points, which are represented by small squares, are called secondaries. Good records of the secondary points are not always obtainable because of 'the after-vibrations caused by the primary pulse, but whenever the secondaries give reliable times these can be used in the same way as the primaries are used. The following discussion refers principally to primaries, but it is intended to cover readable secondaries as well, since there is no essential difference between secondaries and primaries except for the order of their arrival.

It may readily be seen that the slope of line 31 is the reciprocal of the. average velocity of a seismic pulse through upper earth layer I. Similarly, the slope of line 3| is in part a reciprocal function of the velocity of a seismic pulse through second layer I2. But the slope of line 3| is also a function of the inclination of the interface between layers I I and I2, as may be seen by referring to Fig. 1. It will be noted that, proceeding to the right from the shot point, as upper layer II becomes thicker, the length of partial ray I3 through the slower medium becomes longer.

Therefore the slope of line 3| is a little steeper than it would be if upper layer II were of uniform thickness. If layer II were progressively thinner in the direction of the survey, the opposite would be true.

Since the slope of line 3| is a function of two mutually independent unknowns, the velocity of siesmic pulses through layer I2 and the inclination of the upper boundary of layer H, the data represented in Fig. 2 are insufllcient as a basis for deductions concerning subsurface profiles.

I 'I'his difliculty has heretofore been met by running-a similar test over the same surveyed line.

in the" opposite direction, obtaining what is known as a reversed profile. A line analogous to line 3| may be plotted from the reversal data. The slope of this new line will be a function of the same seismic velocity and the opposite inclination of interface as the slope of line 3|; therefore, the apparent velocities as deduced from line 3| and its reversed counterpart may be averaged to obtain the true velocity of siesmic pulses through layer I2, and the difierence between the apparent velocities is a measure of the inclination of the interface between layers II and I2. I

In Fig. 2, point 35 at the intersection of lines 21 and 3| is referred to as the critical point and its abscissa 36 is referred to as the critical distance. The critical distance in many cases is the shortest distance at which useful records of refracted pulses may be obtained, and it is also (when interpreted in view of the velocities of siesmic pulses through layers II and 82) an indication of the thickness af layer II. Point 31, at the intersection of lines 3| and, is known as the second critical point" and its abscissa 38 is called the second critical distance. This secondcritical distance marks the greatest distance at which seismometer records useful for plottin'gline 3|, and therefore useful for indicating the profile of the interface between layers II and I2, may ordinarily be obtained.

Although the primary points 3333 obtained at seismometer stations more distant than the second critical distance are very useful for indicating the nature of the interface between layers I2 and I3, they are not useful for indicating the nature of the interface between layers II and I2, which alone is considered in the following discussion. For this reason, when terms such as the most distant useful seismometer station are used in the following description and in the appended claims, they will be understood as referring to the last seismometer station which yields a readable point on the line characteristic of the interface particularly being studied, whether or not this station is the most distant actually set up.

It will be understood that there are various refinements, such as the allowance for the increase of seismic velocity with depth within a single layer, which are not described herein. These refinements, which are known to those skilled in the art, are of considerable importance in actual practice, but they are not necessary to an understanding of the present invention.

Referring now to Fig. 3, which illustrates my invention, the line IIII represents the surface of the earth. The portion of the figure below this line is a sectional diagram like Fig. l, in which the two earth layers III and 2 are shown, together with certain ray paths of seismic pulses. The portion of the figure above line I I0 is a chart of the same general nature as Fig. 2. As in Fig. 2, observed primary points are represented by small circles. The survey illustrated in Fig. 3 is an example of the type (which seemes to be the more usual) in which conditions are such that no useful points other than the primaries are obtainable.

My method begins in precisely the same manner as the heretofore-known method reviewed above. A charge of explosive is detonated at shot point I4, and the seismometer records of the pulse thereby created are plotted to form lines I21 and I3 I, which are exact counterparts of lines 21 and 3|, respectively, shown in Fig. 2. But I do not run another test in the opposite direction over the same ground, since I have devised a better method of obtaining information suflicient for resolving the indeterminateness of the results yielded by the first explosion.

I have indicated at 40 the position of the useful seismometer station which is most distant from shot point I I4. From this position I measure back toward shot point |I4 a distance 4|, which distance is (preferably) about equal to critical distance I33 or (permissibly) somewhat greater than the critical distance. When such secondary points as are illustrated at 28 in Fig. 2 are readable, distance II is taken as equal to the distance from the shot point to the seismometer station yielding the nearest of such points, or, if desired, a somewhat greater distance. In the appended claims, the minimum for distance 4|, as here defined, is referred to as the distance from the shot point to the "nearest useful seismometer station, although it is to be understood that stations too near to yield readable points on line III are useful for indicating the characteristics of the uppermost layer III. Measuring out distance 4| determines point Illa, which is the location of the second shot point. A second test is run from point Illa in the same manner and in the same direction, yielding a second pair quired for a refracted seismic pulse to travel along the ray path Illa-H9, in either direction.

Furthermore, the ordinate of the last useful primary point, 43, is the time required for a seismic pulse to travel, in either direction, along the ray path H1-l l8-I 19. Therefore, if point 42 is projected back to the vertical erected at H4a, and if point 43 is projected back to the vertical erected at point H4, a new pair of points 44 and 45 is obtained, which determines a new line 46. Line 46 is precisely the same reversed profile line which would be obtained by firing a charge of explosive at point 40 and detecting the pulse by seismometers located at points H4 and H4a if the operators were fortunate enough to find points 44 and 45 to be primaries or readable secondairies. As above indicated, the slope of line 46 is sufiicient to resolve the indeterminateness of the information plotted as line l3l Similarly, from the next last useful seismometer station," 40a, a distance 4la. equal to (or slightly greater than) critical distance I360. is measured back, and another shot point, H4b, is thereby located. The operations are thus repeated, as indicated by analogy of reference numerals, for the entire length of the profile which it is desired to survey.

It will be understood that the particular geometric method of plotting deduced reversed profile data shown in Fig. 3 is merely exemplary; I have chosen it in order to illustrate more clearly the relation between my method and the method used heretofore, in which reversal data" were obtained by an actual reversed run. An alternative method of plotting results is indicated in Fig. 4. In this method, point 42 is found as shown in Fig. 3, but points 42 and 43 are not projected in the same manner. Instead, a new point 244 is constructed, having the same ordinate as point 42 and an abscissa equal to distance 4i. A line 246 is then drawn between points 43 and 244. Line 248 expresses the same information as'line 46 of Fig. 3, and its position makes. it somewhat preferable for quick mental interpretation.

Distance 4| should be about the same as distance l36a; if it is much shorter, such great extrapolation is required for locating point 42 that the method becomes undependable, and if distance 4| is much greater than l36a some of the economy of labor and materials attainable by the new method is lost. But distance l36a is unknown at the time distance 4| is laid out; I

therefore use the previous critical distance, I36,

as a guide, since that distance is the best available approximation of distance 136a. It will be understood that in particular cases, such as when it is desired to avoid surface improvements which might be damaged by an explosion, distance 4| may be laid {out considerably greater than the distance required for greatest economy; this has the effect of moving point 44 along line 46 toward point 45, but it does not substantially impair the validity of the results.

As indicated in Fig. 3, when the second and subsequent shot points, H4a, H4b, etc., are fired,

only one seismometer station nearer to the shot point than the probable critical distance is used. This is because one point, in addition to the location of the shot point, is sufficient for determining lines l21a, l2'lb, etc., which are very nearly straight. The resulting economy of'time and labor is of considerable importance; however, it will be apparent that, if it is desired to give particular study to departures from homogeneity within layer HI, additional stations nearer to the shot point may be set up.

Another variation is to finish the profile by an actual reversal shot. That is, considering the right-hand end of Fig. 3 to designate the end of the line it is desired to survey, instead of firing a shot at the location indicated as H40, the shot.

could be fired at point 40b with the seismometer stations located back along the line of survey toward the point of beginning. In this manner. point 441) would be found directly rather than deduced from an imaginary reversal.

In the prior art, the reversal lines 46, 460, etc., had to be determined by placing actual shot points at 40, 40a, etc., which involved great additional expenditure of time, materials, and labor. But by my particular arrangement of shot points and seismometer stations, it is possible to deduce the reversal data without actually shooting the reversals. Other arrangements, random or systematic, will not furnish the required data as inexpensively.

A particularly valuable advantage of the present invention resides in the continuity of the data for the determination of the interface between layers HI and H2. Such continuity of coverage was practically never obtained in the prior art, because the cost of obtaining it by the heretoforeknown method was virtually prohibitive. A further advantage of the new method is found in the fact that the work is so arranged that the entire crew moves always in one direction. This features is always advantageous, and it is particularly important when working over a terrain where transportation is diflicult.

I claim as my invention:

1. In a refraction method of seismic geophysical surveying which includes the creation of seismic pulses at a series of points and the detection of said pulses at seismometer stations spaced along the line of survey, the improvement which ful seismometer station back along the line of survey toward the point of creation of said pulse a distance not substantially less than the distance from the point of creation of said pulse to the nearest useful seismometer station as indicated by the records of said pulse; creating a subsequent seismic pulse at .the location so determined; recording said subsequent pulse at seismometer stations placed along the line of survey in the direction away from first said point of creation of a seismic pulse; and repeating the recited operations until the line of survey is covered.

2. In a refraction method of seismic geophysical surveying which includes the creation of seismic pulses at a series of points and the detection of said pulses at seismometer stations spaced along the line of survey, the improvement which comprises: after the creation and recording of each seismic pulse, measuring from the "last usesurveytoward the point at creation of said; pulse 7, l i I a 5 distance; netj substantially; iess: than the :dis-: 7 tance, from the point ofcrsatlon- Qf, saidpulse r to the nearestuseful se'ismometer stati n'n as 1 indicated by the; records; of said; pulse: creating a; subsequentseismic pulse at, the lotiationso (16-? I termined; recording; said. subsequent pulse I at y 1 s z smo met r s a i ns :p acdl a on t e 1km of V V surveyin the direction away from firstsaid'point v 01' creatio njoi a seismicpulse; repeating the re- '10 I *determinateness of theunreversed data; 

